The present invention relates generally to light emitting diodes (LEDs) and more particularly to LEDs comprised of a nitride semiconductor light emitting component and a conductive group II-V semiconductor, preferably satisfying a formula Zn1−a−b−cMgaCdbBecO1−p−qSpSeq, wherein a=0˜1, b=0˜1, c=0˜1, p=0˜1, and q=0˜1.
Most LED structures composed of group III nitride semiconductors are grown on sapphire substrates. The drawback of sapphire in this application is that it is electrically an insulator. As a result, a positive electrode 100 and a negative electrode 500 of an LED structure can not sandwich a light-emitting component 300 and have to be made on the same side of a sapphire substrate 401, as typically shown in FIG. 2, which is known as lateral injection structure.
LEDs of the said lateral injection structure are known to have low wall-plug efficiency, which results from three major facts. First, the effective current injection area or a light emitting area 310 is restricted by the area of the positive electrode 100. In the lateral injection structure, the positive electrode 100, having poor transparency, can only be fabricated to cover a small portion of a p-type nitride semiconductor 301, or most of the light will not be extracted. Because the p-type nitride semiconductor 301 is a poor semiconductor, significantly less conductive than an n-type nitride semiconductor 303, the injected current can not be effectively spread laterally and the current injection is only restricted under the positive electrode 100. Second, a significant amount of output light is blocked by the electrode 100. Third, LEDs of the lateral injection structure have an additional series resistance, caused by a current crowding effect in a thin n-type nitride semiconductor 303 as the electrical current passes by laterally. This inevitably results in a significant reduction of light emission efficiency due to thermal effects.
If a highly conductive, transparent material replaces the insulative sapphire substrate 401 and another highly conductive, transparent material is attached onto the top of the less conductive, p-type layer 301, then a vertically structured LED is generally formed as shown in FIG. 3 (although, it should be emphasized, not as later specifically discussed with respect to FIG. 3). Compared to the said lateral injection structure, the vertically structured LED has an enlarged effective current injection area or light emitting area 310 and an enhanced ratio of the output light to the light blocked by the positive electrode 100. Also there is no more considerable said additional series resistance associated with the lateral transport in the n-type nitride semiconductor 303. Therefore, the vertically structured LED has a significantly improved wall plug efficiency. The effective current injection area or light emitting area 310 herein has the same normal as that of the sapphire substrate surface.
Theoretically, GaN could be an ideal substrate to replace the insulator, sapphire substrate 401 for the said vertical LED structure. However, high crystalline, high conductive GaN substrates are not available with low cost. Moreover, homoepitaxy growth of nitride LED structure on GaN substrates has so far been considered very challenging.
SiC substrate has been used as a conductive substrate for the said vertical LED structure. Yet, the bulk growth of SiC is very difficult since the melting point of hexagonal SiC is over 3000° C. and, consequently, SiC substrates are expensive. There are also significantly large a and c axis lattice parameter mismatches between SiC and GaN, 3.6% and 94%. respectively.
Silicon could also be another alternative conductive substrate for vertical LED. However, it is a narrow bandgap semiconductor and known to significantly absorb ultraviolet, violet, blue, green, yellow, red, and near infrared light. Therefore nitride LEDs formed on silicon substrates have low efficiency.